The present invention relates to paper machines.
In particular, the present invention relates to paper machines particularly suited for manufacturing tissue paper.
The former section of the machine of the invention includes a smooth forming roll around which inner and outer endless fabric means, each in the form of a wire or felt, are guided to define at this forming roll a twin-wire former wherein the inner fabric means is situated between the smooth forming roll and the outer fabric, with dewatering of the web taking place at the twin-wire former primarily as a result of centrifugal force while the web is sandwiched and compressed between the inner and outer endles fabric means, the machine preferably having a hydraulic headbox.
It is known to manufacture tissue paper by forming a web on a fairly short wire section resembling a normal planar wire where a headbox applies the pulp stock onto a breast roll, which frequently has an open interior and is provided with an internal suction. In such machines the web travels while supported on the wire past conventional dewatering elements such as table rolls, deflectors, foil lathes, suction boxes, and a suction roll, each of which removes water from the stock web. At the discharge end of the wire section the partially dried web is transferred onto a so-called pick-up felt which supports and conveys the web to undergo further drying in the press and dryer sections of the machine.
In certain known machines the above planar wire has been entirely omitted. With constructions of this type the formation of the web takes place entirely on the suction breast roll which is wire-coated and from which the web is transferred directly onto the pick-up felt.
With constructions as referred to above, there is the drawback, among others, that the upper speed limit of the paper machine will be approximately 1500 m/min, because the draining pressure reaches such a great value that it causes the web to adhere undersirably to the wire, with consequent difficulties of the detachment of the web from the wire.
There are several known twin-wire formers intended for the manufacture of tissue paper. Most of these twin-wire formers are so-called full throat formers, but these structures have in general the drawback that as a result of the throat- forming process only a poor web formation is achieved. With such constructions difficulties are encountered in transferring the web onto the transporting wire. Also, control of dewatering is difficult.
With respect to patent literature pertinent to the present invention, reference may be made, by way of example, to Canadian Pat. No. 968,601, in which a certain type of pick-up press is disclosed, in the general field of the present invention. Reference also may be made to U.S. Pat. Nos. 3,378,435 and 3,537,954, of which the first provides the so-called Crescent former. Closely related to these patents are U.S. patent application Ser. No. 730,444 filed May 20, 1968. Also, reference may be made to British Pat. No. 1,244,040, in which the so-called Papriformer of AB Karlstads Mekaniska Verkstad is disclosed.
The present invention is a further development of what is shown in U.S. Pat. No. 3,846,232 and U.S. application Ser. No. 493,704 filed July 31, 1974, now U.S. Pat. No. 3,997,390. With respect to these constructions reference may be primarily made to the single-wire initial part of the former disclosed therein, as well as to the possibilities of controlling different process variables in a manner to which the practical papermaker is already accustomed on Fourdrinier paper machines. In this connection, the most important of these process variables are the speed of discharge of the stock jet relative to the speed of travel of the wire, the angle at which the stock jet meets the wire, and the rate at which water is drained from the suspension deposited on the wire.